Plant Genome IV Conference Plant Genome IV Conference
We are analyzing induced and spontaneous mutants of lettuce that have lost resistance to Bremia lactucae, the causal fungus of downy mildew. Mutations were induced using fast neutron (FN) irradiation and ethyl methane sulfonate (EMS) in the lettuce cultivar Diana that contains the resistance genes Dm1, Dm3, Dm7 and Dm8, located in each of the three major clusters of resistance genes. Sixteen Dm mutations were detected in 2,211 FN-treated M2 families. Nine of these mutants showed loss of Dm3 specificity and were missing linked molecular markers, indicating deletions in the region. Twenty-five Dm mutations were recovered from 4,000 M2 families treated with EMS. One mutant showed loss of both Dm1 and Dm3. In contrast to the FN deletion mutants, the EMS mutants showed a range of susceptible phenotypes, including profuse sporulation, and sparse sporulation accompanied by local necrosis. In addition, numerous M2 families showed local or regional necrosis without sporulation. All molecular markers were present in the EMS mutants. All mutations segregated as single recessive alleles. Complementation tests are in progress. We have also been isolating spontaneous mutants from two different populations to examine the natural mutation rate of Dm genes. Six Dm mutations were recovered from 4,600 S2 families of Diana. Four of these mutants showed altered Dm3 specificity and, similar to the FN-induced mutants, were missing linked molecular markers. Three spontaneous mutants were recovered from 8,500 F1 individuals derived from the cross, Kordaat (Dm1, Dm3, Dm4) x Calmar (Dm7, Dm8, Dm13). Of these, one is missing molecular markers in the Dm3 region. Genetic analyses are currently underway and preliminary results indicate single gene, recessive mutations. These mutant genetic stocks, both induced and spontaneous, are being used to investigate the structure and nature of resistance gene clusters in lettuce. The characterization of these mutants aids in the localization of genes in the cluster and allows investigation of mechanisms generating new resistance specificities.